Southwick, P. L., et al., J. Org. Chem., 39, 3351 (1974), described the preparation of the reactive nitro enol ether 1-cyclohexyl-3-ethoxyl-4-nitro-2-oxo-3-pyrroline(6a), as well as some applications of this compound as a reagent for the introduction of a reversible amino protecting group (the 1-cyclohexyl-4-nitro-2-oxo-3-pyrrolin-3-yl or "c-NOPY" group where "NOPY" stands for 4-nitro-2-oxo -pyrrolin-3-yl). Applications of the NOPY blocking group in simple examples of peptide synthesis and protein modification have been described in Southwick, P. L. ibid. and Negri, D. J.; Southwick, P. L.; Brown, W. E., Biochem. Biophys. Acta, 579, 31 (1979).
Advantages of the NOPY-group are its ease of introduction (short reaction times in partially aqueous media at ca. 25.degree. C. and pH 7.5-9), ease of removal (treatment with ammonia or aqueous base at ca. 25.degree. C.), and the convenience with which it is monitored through its high absorptivity at 367-385 nm and visibility on thin-layer chromatography plates with 254-nm fluorescent indicator.
For many applications, however, it would be useful to have a new blocking reagent that would be more water soluble and which would introduce a blocking group that adds only minimally to the hydrophobic character of a resulting protected amino acid or peptide derivative. A reagent of the NOPY type with cyclohexyl replaced by a smaller group was thought likely to have such properties.
The reagent 9-fluorenylmethyl chloroformate also introduces a blocking group which is removable under mild alkaline conditions. It is, however, water-insoluble, as is a previously introduced congener of our new reagent. The presence of the blocking group introduced by the 9-fluorenylmethyl chloroformate reagent is less readily determined spectroscopically.
U.S. Pat. No. 4,460,501 also discloses blocking groups for protecting amine groups.